05000424/LER-2014-002
| Vogtle Electric Generating Plant - Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4242014002R00 - NRC Website | |
A. REQUIREMENT FOR REPORT
This report is required per 10CFR 50.73(aX2)(iv)(A) due to an unplanned manual actuation of the Reactor Protection System (RPS) and automatic actuation of the Feedwater Isolation (FWI) Engineered Safety Features Actuation System (ESFAS).
B. UNIT STATUS AT TIME OF EVENT
Mode 1, 28 percent rated thermal power.
C. DESCRIPTION OF EVENT
On 4/12/14 at 20:08, Unit 1 Reactor was manually tripped from 28 percent power. The manual trip occurred during power ascension following the 1R18 refueling outage. Control room operators received a Loop 1 Train B main steam isolation valve (MSIV) trouble annunciator followed by the MSIV not fully open annunciator. Control room operators recognized steam generator 1 level and loop 1 steam flow lowering and manually tripped the reactor.
The direct cause of the event was a failed 0-ring on the Loop 1 Train B MSIV lower manifold-to- cylinder mating surface resulting in a loss of hydraulic oil pressure. The root cause was misalignment of the lower manifold-to-cylinder mating surface during valve reassembly. Post- event test results from an independent laboratory concluded the pinched 0-ring occurred as the lower manifold and the cylinder were connected and bolted together. The MSIV drifted closed to its fail safe position as hydraulic pressure lowered. Unit 1 was stabilized in Mode 3. Plant systems responded as expected with decay heat removal via Auxiliary Feedwater and steam discharge to the Main Condenser. Plant systems responded as expected and there were no adverse effects on plant safety or on the health and safety of the public. Unit 2 was unaffected.
D. CAUSE OF EVENT
The direct cause of the event was a failed 0-ring on the Loop 1 Train B MSIV lower manifold-to- cylinder mating surface. This resulted in a loss of hydraulic oil pressure causing the MSIV to fail closed. The 0-ring was pinched during reassembly of the lower manifold and cylinder following valve maintenance.
E. SAFETY ASSESSMENT
The MSIV failed to the closed position thereby fulfilling its designed safety function. Unit 1 was stabilized in Mode 3. Plant systems responded as expected with decay heat removal via Auxiliary Feedwater and steam discharge to the Main Condenser. There were no adverse effects on plant safety or on the health and safety of the public. This incident is of very low safety significance.
F. CORRECTIVE ACTION
Maintenance instructions will be provided to ensure proper alignment of the lower manifold-to- cylinder mating surface to prevent damaging the 0-ring during installation or valve reassembly.
G. ADDITIONAL INFORMATION
1) Failed Components:
Component: ISV-Isolation Valve 2) Previous Similar Events:
Licensee Event Report 50-425/90-008-000, "Manual Reactor Trip Following MSIV Closure Due to 0-ring Failure' A similar event occurred at Vogtle Unit 2 in 1990. The cause for the MSIV closure was the failure of an 0-ring sealing a connection on the non - pump side hydraulic manifold. Although failure modes were similar, rotary or oscillatory movement during re-assembly was not identified as a primary contributor.
3) Energy Industry Identification System Code:
Main/Reheat Steam System - SB NAG FORM 306A (024014)